Python syntax in Lisp and Scheme

[gregm at cs.uwa.edu.au]

In comp.lang.functional Erann Gat <my-first-name.my-last-name at jpl.nasa.gov> wrote:
:> I can't see why a LISP programmer would even want to write a macro.
: That's because you are approaching this with a fundamentally flawed
: assumption.  Macros are mainly not used to make the syntax prettier
: (though they can be used for that).  They are mainly used to add features
: to the language that cannot be added as functions.

Really? Turing-completeness and all that... I presume you mean "cannot
so easily be added as functions", but even that would surprise me.
(Unless you mean cannot be added _to_Lisp_ as functions, because I don't
know as much as I'd like to about Lisp's capabilities and limitations.)

: For example, imagine you want to be able to traverse a binary tree and do
: an operation on all of its leaves.  In Lisp you can write a macro that
: lets you write:
: (doleaves (leaf tree) ...)
: You can't do that in Python (or any other langauge).

My Lisp isn't good enough to answer this question from your code,
but isn't that equivalent to the Haskell snippet: (I'm sure
someone here is handy in both languages)

doleaves f (Leaf x)     = Leaf (f x)
doleaves f (Branch l r) = Branch (doleaves f l) (doleaves f r)

I'd be surprised if Python couldn't do the above, so maybe doleaves
is doing something more complex than it looks to me to be doing.

: Here's another example of what you can do with macros in Lisp:

: (with-collector collect
:   (do-file-lines (l some-file-name)
:     (if (some-property l) (collect l))))

: This returns a list of all the lines in a file that have some property. 

OK, that's _definitely_ just a filter: filter someproperty somefilename
Perhaps throw in a fold if you are trying to abstract "collect".

: DO-FILE-LINES and WITH-COLLECTOR are macros, and they can't be implemented
: any other way because they take variable names and code as arguments.

What does it mean to take a variable-name as an argument? How is that
different to taking a pointer? What does it mean to take "code" as an
argument? Is that different to taking a function as an argument?

-Greg